I started a Garmin Coach training plan and later quit the plan because I change my goals. Now it's listed in my "Completed Plans" list and there is no way to delete it.
Why would a plan I quit be listed as Completed? Makes no sense.
EDIT: April 17, 2020, with Garmin Connect app version 4.29, the ability to remove incomplete Garmin coach training plans from the completed plans list.